Course: Plant Physiology - PowerPoint Presentation

Slide1

Course: Plant Physiology

Code: 308 Bot

Students: Third year Botany

Lecture

2

(Plant Mineral Nutrition)

Dr.

Taha

Mohamed El-

Katony

Slide2

Absorption and translocation

The path of ions across the root

Absorption of salts by roots is both passive and active.

The movement of ions into the AFS (

apoplasm

) is passive (by free diffusion).

Ions can move freely in the

wet cell walls (

apoplast

) of the r

oot as far as the endodermis.

Further penetration is retarded by the

C

asparian

strip.

The continuum of cytoplasm and the connecting

palsmodesmata

is called the

symplast

.

Slide3

The endodermal cells by virtue of the

Casparian strip present a barrier to the passive diffusion of ions.

The protoplast of endodermal cells is tightly attached to the

Casparian

strip.

Ions can not pass through the walls of the endodermal cells.

The only route available is through the protoplast.

Slide4

How can ions be delivered into xylem?

It was supposed that there is a gradient of decreasing O

2

and increasing CO

2

levels from the cortex to the stele.

The living cells adjacent to the xylem vessels therefore possess low metabolic activity, are leaky and lose salts to the lumen of xylem vessels.

Diffusion of salts back through the impervious

Casparian

strip is not allowed and there is a unidirectional loss of salt into the

lumina

of the xylem vessels.

Slide5

Slide6

Circulation of salts

Salts delivered to the xylem ducts of the root are

translocated

upward to the shoot and, once there, are distributed and redistributed throughout the plant.

Salts deposited in the leaves are withdrawn prior to abscission and

translocated

to the reproductive organs or younger leaves.

Circulation takes place in the vascular tissues.

Slide7

By the use of radioactive tracers several pathways for the translocation of salts were discovered:

1- in the xylem,

2- in the phloem,

3- laterally between the two tissues and

4- outward from the leaves.

Slide8

Translocation of salts in xylem

Experimental evidence

1-

Ringing experiment:

The

upward translocation of salts is unimpeded by removal of a ring of phloem near the stem base.

2- L

arge amounts of salts occur in the xylem sap.

3-

32

P

travels upward to plant tip faster under high transpiration (bright sunlight) than under low transpiration.

4-

If transpiration by a leaf is diminished by covering the leaf with a polyethylene bag, translocation of minerals to that particular leaf is reduced considerably.

Slide9

5- Dissection experiment with radioactive tracers:

A: Procedure

i- The bark and xylem along a 9-inch length of a willow stem were separated and a strip of impervious waxed paper was inserted between them.

iii- The continuity of bark and xylem was undisturbed, and the plant was left intact.

iv- The plant was allowed to absorb

42

K for 5 hours.

v- Sections of the treated and intact areas of the stem were analyzed for

42

K.

Slide10

Slide11

Section

Stripped

branch

Unstripped

branch

42

K in bark

(

ppm

)

42

K in wood

(

ppm

)

42

K in bark

(

ppm

)

42

K in wood

(

ppm

)

SA

53.0

47

64

56

S6

11.6

119

S5

0.9

122

S4

0.7

112

87

69

S3

0.3

98

S2

0.3

108

S1

20.0

113

SB

84.0

58

74

67

Slide12

B: Results and conclusion

1-

42

K

translocates

upward in the xylem.

2-

Lateral interchange

of

42

K between phloem and xylem (through cambium) occurs within the intact region but

further translocation

either upward or downward in the

phloem

is

slow

.

3- The small amounts of

42

K in the bark along the stripped area suggests

that little

translocation

takes place in the

phloem

tissue.

Slide13

Lateral translocation of salts

T

he above experiment revealed:

1- upward translocation of salts in xylem

and

2- lateral movement between xylem and phloem through cambium.

Cambium regulates the amount of salts carried up in the transpiration stream.

If the upward movement of salts were not regulated, certain areas of the plant

(shoot and root tips) would

not be accommodated.

The cambium position allows - both

metabolically

and

physically

-

re

gulation of the upward, lateral and downward movement of salt.

Slide14

The active accumulation of salt by the cambial cells acts as a deterrent against an indiscriminate sweep of salts upward in the transpiration stream.

The lateral movement from xylem to phloem correlates inversely with element content in the phloem

high concentration of an element in the phloem, slows down the lateral translocation into the phloem and low concentration

enhances lateral movement.

Slide15

Translocation of salts in the phloem

The initial upward movement of salts occurs in the xylem.

But upward movement may occur also in the phloem.

Evidence on upward movement:

1- Ringing bark high up in the stem

retarded s

tem tip growth.

But ringing at stem base has no effect on salt nutrition of the whole plant including stem tip.

Thus, the influence on stem tip growth was because of the blockage of salts moving out of the lower leaves and transported upward in the phloem and not because of the root-absorbed salts.

Slide16

Evidence on downward movement:

2- Dissection experiment with Radioactive tracers.

Procedure

:

A- The bark was separated from wood

by waxed paper

immediately below leaf petiole of a cotton plant.

B- Bark and wood were left intact.

C-

32

P was injected into leaf blade just above the separated area.

D-

One hour

later, sections of stem were analyzed for

32

P.

Slide17

Slide18

Section

Stripped

plant

Unstripped

plant

32

P in bark

(mg)

32

P in wood

(mg)

32

P in bark

(mg)

32

P in wood

(mg)

A

1.11

I

0.485

0.100

0.444

C

0.610

S1

0.554

0.064

0.160

0.055

S2

0.332

0.004

0.103

0.063

S3

0.592

000

0.055

0.018

S4

0.228

0.004

0.026

0.007

B

0.653

0.152

Slide19

Results and conclusion

1- Salts entering the main vascular stream from leaf move primarily in a downward as well as upward direction in the phloem.

2

- Lateral transport between the vascular tissues takes place where the phloem and xylem are not separated.

4- Both tissues may thus be involved with the upward translocation of mineral salts moving out from leaves.

Slide20

The movement of salts in the phloem is bidirectional.

This bidirectional movement might occur:

1- Simultaneously in the same sieve elements.

Or

2- In two different phloem channels, one toward the tip and the other toward the base of the plant.

Slide21

Outward movement of salts from leaves

In the

leaves of deciduous plants,

mineral salts move out of the leaf

just prior to abscission.

The mineral nutrients moving out of leaves are N, K, P, S,

Cl

and

(

Mg and Fe).

Those remaining include

Ca

, B,

Mn

and Si.

Mineral nutrients move out of leaves primarily in the phloem.

When

32

P was introduced to leaves at different levels on the plant: P from base leaves moves downward toward the root

while P from top leaves moves upward to tip.

Slide22

Circulation and reutilization

Minerals are taken up in the transpiration stream.

Move primarily in the xylem vessels to the leaves.

Excess quantities are withdrawn from leaves

and

re-

translocated

downward in the phloem.

Then they could be laterally transported into the xylem

w

here upward translocation could take place again.

Elements such as N, K and P move readily in this circuit.

Calcium ascends the stem but does not relocate in the phloem.

Slide23

Phosphorus

P is highly mobile in the plant and it is in continuous circulation.

P mobility is an essential feature of plant growth.

P is needed in such metabolic schemes as photosynthesis, starch synthesis, glycolysis, and the synthesis of fats and proteins.

P is thus needed at various points in the plant.

A pool of P in a useable form is maintained throughout the plant in a relatively uniform concentration.

Slide24

Sulfur

Sulfur is mobile in plants but to a lower extent than P.

This is because of its rapid incorporation into metabolic compounds.

Absorbed *S rapidly ascends in the xylem to the leaves.

Within 24 h most of the *S was found in the younger leaves.

The older leaves having lost their S to the young actively growing leaves.

S is a constituent of protein and protein synthesis occurs heavily in the younger leaves.

S

, then, is freely mobile in the plant but is rendered immobile rather quickly in metabolic reactions.

Slide25

Iron

Mobility of iron in the plant

depends on :

1-

Fe concentration

of the plant tissues

.

Being highest at low

Fe concentration in the plant

and

decreases at high concentration.

2- P

availability.

High

P concentrations in the plant tissues render iron immobile in the veins of the leaf

.

3- pH of the rooting medium.

A

pH of 4

gives

high iron

mobility and mobility

decreases

at pH of

7.

Slide26

Calcium

Absorbed *

Ca

ascends in the transpiration stream to the different areas of the plant.

Ca

is immobile in the phloem, and once delivered by the transpiration stream, it remains stationary

.

Slide27

In conclusion:

1-

T

here are four general directions of ion movement in the plant:

upward, downward, lateral and outward.

3-

The upward translocation of salts takes place primarily in the xylem, but also in the phloem.

4- Downward movement takes place in the phloem where upward movement also occurs.

5- Movement of salts in the phloem is, thus, bidirectional.

6- Lateral movement occurs between xylem and phloem through the cambium.

7- Movement of salts out of leaves is common, especially prior to abscission, and occurs in the phloem.